The present invention relates to satellite-based communications systems which utilize weather information for areas to be serviced by the satellite beams to adjust the effective isotropic radiated power of a portion of the beams and minimize the net RF power required for transmission.
Satellite-based communications systems are well represented in the prior art. In the typical satellite communication system, signals are transmitted between one or more feeder Earth stations and a satellite. The satellite receives the signals and then transmits them down to end user stations with the use of one or more repeaters.
Current satellite transmission systems include fixed gain amplifiers with their operating points determined during the design of the communication system to achieve predetermined system performance goals. The transmission systems are designed at their inception with sufficient transmit power to overcome a low probability precipitation event which may otherwise have an adverse affect on signal reception at a particular location within the coverage area of the satellite. For example, a system requiring 99.9% link availability, with an accompanying 3 dB of rain attenuation in the transmission path, will be designed with 3 dB additional satellite radio frequency (RF) transmit power applied to or available for the given antenna beams. This RF transmit power is constant and cannot be adjusted over the lifetime of the satellite. Furthermore, the transmit power is applied to beams covering the entire broadcast area, which may be several times larger than a localized precipitation event. Consequently, satellite power requirements are oversized to achieve desired link availability and customer satisfaction during worst-case weather conditions. This sizing and utilization of RF transmit power is extremely inefficient, particularly where the satellite has a broad coverage area.
One prior art approach utilizes a low frequency beacon or reference beam signal to provide an indication of signal loss attenuation. The low frequency reference beam is less susceptible to attenuation and is used as a feedback signal to adjust the power of the uplink signal. As such, the fixed gain amplifiers receive a stronger signal which in turn results in a stronger downlink signal. Similar to the approach discussed above, this approach provides increased power to the entire broadcast region rather than selectively increasing power only in the region affected by a precipitation event.
Accordingly, it is an object of the present invention to provide a system and method for adjusting RF transmit power for a portion of the coverage area in response to signal attenuation.
It is a further object of the present invention to provide a system and method for pooling the total RF power applied to all spot beams and redistributing transmit power according to localized downlink signal attenuation.
It is still a further object of the present invention to provide a system and method for adjusting the antenna beam pattern gain to compensate for signal attenuation due to changes in the weather.
Yet another object of the present invention is to provide a system and method for reducing net RF transmit power while achieving desired communications performance as measured by data rate and link availability.
An additional object of the present invention is to provide a system and method for modifying the effective isotropic radiated power using a combination of RF power attenuation control and dynamic beam shaping of the transmit antenna beam pattern to compensate for precipitation induced signal attenuation.
In carrying out the above objects and other objects and features of the present invention, a system for controlling the transmit power to selectively increase power within a portion of a coverage region acquires signal loss information based on weather maps and/or actual downlink signal measurements. In one embodiment, radar reflectivity data is acquired and then translated via a series of analytical models into rain attenuation values. These analytical models may include, but are not limited to: 1) the satellite Earth geometry (elevation, slant range); 2) radar measurement parameters such as reflectivity, differential reflectivity, and/or differential phase; 3) models for raindrop size and orientation; and 4) site-specific information such as latitude, temperature, altitude, and the like. The calculations are performed on a computer located either within a ground station or on a satellite.
In the alternative, downlink power measurements may be made by a network operations and control center (NOCC). The NOCC is preferably capable of adjusting the number of sites polled and/or the monitoring or polling rate depending upon weather conditions in the localized regions within the satellite coverage area. In one embodiment, data from multiple sites within a single spot beam coverage are merged to create a single transmission loss or required minimum EIRP for the beam. Data from sites within an area coverage beam may be used to create an attenuation map for pattern optimization. The NOCC is preferably capable of processing terminal measurements and formulating a precipitation attenuation map similar to that which would be provided from a weather data source.
The present invention utilizes either the weather radar map, the downlink power measurement strategy, or a combination of both, to generate commands to selectively control the satellite transmit subsystem to adjust beam attenuation and beam patterns to increase signal transmit power within localized regions experiencing downlink power attenuation while reducing or maintaining transmit power to other localized regions.
The present invention may be implemented in any satellite transmit subsystem capable of adjusting RF transmit power and/or antenna gain patterns to vary EIRP to selected regions on the Earth. The EIRP adjustment required for transmission loss compensation will vary depending on the particular satellite transmit subsystem. For spot beam satellite coverage, the antenna gain pattern is fixed for each spot beam and EIRP is adjusted by modifying RF transmit power. In the case of broad area coverage, the antenna gain pattern is reshaped.
The advantages accruing to the present invention are many. The present invention increases link availability by reallocating RF transmit power to localized areas when required to allow for a lower net transmit power under normal weather conditions. Total RF transmit power is conserved with a corresponding reduction in DC power and thermal dissipation requirements on the satellite. Depending upon the particular application, this enables either increases in satellite functionality and/or increases in total capacity of the satellite communication system.
The above objects and other objects, features and advantages of the present invention will be readily appreciated by one of ordinary skill in the art from the following detailed description of the best modes for carrying out the invention when taken in conjunction with the accompanying drawings.